1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device such as a nitride semiconductor laser device, to a method of fabrication thereof, and to a semiconductor optical device provided with a nitride semiconductor laser device as a light source.
2. Description of the Prior Art
There have been test-fabricated semiconductor laser devices that lase in an ultraviolet to visible region of the spectrum by using nitride semiconductor materials as exemplified by GaN, AlN, InN, and compound crystals thereof. An example of such a semiconductor laser device is reported in Japanese Journal of Applied Physics, Vol. 39 (2000), pp. L647-650. According to this document, a nitride semiconductor laser device is formed on top of a GaN substrate in the following manner. First, on top of the GaN substrate, an SiO2 mask pattern is formed that has stripe-shaped openings formed periodically therein, and then, further on top thereof, a layered structure of a nitride semiconductor is formed that has stripe-shaped waveguides (ridge-stripe structures).
The substrate is reported to be fabricated through the following procedure. On primer GaN having a SiO2 mask pattern formed thereon, the SiO2 mask pattern having stripe-shaped openings formed periodically therein (with a period of 20 μm), a 15 μm thick GaN layer is formed by MOCVD (metalorganic chemical vapor deposition) to produce a wafer with a flat surface. This is a technique called ELOG (epitaxially lateral overgrown), which exploits lateral growth to reduce defects. Further on top, a 200 μm thick GaN layer is formed by common HVPE (hydride vapor phase epitaxy), and then the primer is removed. Now, the fabrication of a GaN substrate is complete. A semiconductor laser as actually produced in this way was estimated to have a life of 15,000 hours at 60° C. and at 30 mW.
One disadvantage of the semiconductor laser device described in the document mentioned above is that the fabrication procedure thereof involves three sessions of crystal growth (primer growth, MOCVD growth, and HVPE growth), and is thus complicated, resulting in unsatisfactory productivity. Another disadvantage is an unsatisfactory laser oscillation life, in particular under high-temperature, high-output conditions (for example, at 70° C. and at 60 mW).
As is the case in the example described in the document mentioned above, typically used as substrates are GaN substrates, which have therefore been researched intensively in many research institutions. However, to date, no semiconductor laser devices have been obtained that offer satisfactorily long lives, and therefore longer lives are now being sought in semiconductor laser devices. It is known that the life of a semiconductor laser device depends heavily on the density of defects (in the present specification, defects denote atomic vacancies, interstitial atoms, dislocations, and the like) inherent in a GaN substrate. However, substrates with low defect density are difficult to produce, albeit said to be effective in achieving longer lives, and have thus been researched eagerly.
For example, Japanese Patent Application Laid-Open No. 2000-223743 discloses a method of producing a nitride semiconductor light-emitting device structure on the top surface of a GaN substrate which is slanted relative to the C plane. This helps to reduce lattice defects in the nitride semiconductor layer formed on the GaN substrate and thereby to achieve a longer useful live.
However, it has been suggested that, with conventional nitride semiconductor laser devices like the one disclosed in Japanese Patent Application Laid-Open No. 2000-223743, it is impossible to obtain satisfactorily long lives when they are subjected to high-output aging over a wide area (or the whole area) on the produced substrate. Moreover, no mention is made of variations in characteristics among individual devices obtained after separation into discrete chips.
As described above, using a nitride semiconductor substrate having a laser structure produced by a conventional crystal growth technique often results in an unsatisfactory life, or in a lower yield rate due to variations in the characteristics of chips.